Cardiac compression has been used to boost a failing heart for many years and in its most simple life-saving form involves the compression of the chest wall of a patient. In an emergency situation, a surgeon may take this one step further by manually compressing a heart that has failed, until recovery or an alternative treatment is instituted.
Of course, not all patients present in an acute state and typically a heart will be damaged over a period of time. This can also result in heart failure, a situation which occurs when the heart fails to maintain sufficient circulation of blood to provide adequate tissue oxygenation. Heart failure is widespread in the community affecting for example, 5 million Americans at any one time. Despite recent advances in cardiology, it remains on the increase.
Mechanical heart assist devices that can be used to boost an ailing heart have the potential to provide a quality of treatment that seriously challenges current treatment options, including heart transplantation. Whilst heart transplantation is effective in patients with severe heart failure, the shortage of donor hearts, the expense of the operation and post-operative care, and the risk of rejection are major drawbacks to this option ever fulfilling community expectations.
Several mechanical devices have been developed, one of which is the subject of U.S. Pat. No. 5,119,804 to Anstadt. This device comprises a cardiac massage cup adapted to fit loosely over a lower portion of a heart. A diaphragm is positioned internal the cup and positive and negative pressure applied to the space between the diaphragm and the cup to alternately inflate and deflate the diaphragm. When the diaphragm is inflated, the heart is squeezed to assist systolic action (ejection of blood from the ventricles of the heart). The diaphragm is deflated to correspond with diastole (relaxing of the heart muscle and filling of the heart pumping chambers with blood). The cup itself is held in place around the heart by a suction force which prevents the heart from dislodging when compressive pressure is applied to the heart.
The requirement that the diaphragm be set inside a cup results in a bulky device which may also cause damage to the heart muscle, coronary circulation and the surrounding tissue.
Variations of the Anstadt cup have been developed including the device subject of U.S. Pat. No. 5,713,954 which describes a cuff to enclose the lower regions of the heart. The cuff comprises a series of closed tubes which may be hydraulically or pneumatically inflated in synchrony with the natural contractions of the heart to reinforce the contractile force required to eject sufficient blood for the needs of the body. Literature reports have shown the enhancement of heart pumping by other currently described cardiac compression devices to be limited to between 10 and 15%.
A drawback of several assist devices is that the right and left ventricular pumping action of the heart is simulated using a single diaphragm. It is well recognised, however, that differences exist between right and left ventricular output and that right and left ventricular pressures are different. Essentially, because the left ventricle is ejecting blood to the entire body it requires a greater force of contraction. Devices with only one diaphragm will not assist to provide optimum output of either the right or the left ventricle. A device designed to address this problem is described in U.S. Pat. No. 5,749,839 to Kovacs wherein the assist device is provided with two independently operated diaphragms within a cup to allow for independent control of the left and the right ventricles. This device does not seem, however, to take into account the difference in curvature between the surface of the left-and right ventricles and uses a diaphragm of the same shape for both ventricles. This would seem to potentially result in a misfit of the device over the heart if used in this manner.
With the cardiac assist devices described above, there must be a means for securing the device to the external surface of a heart. Securement may be achieved by applying suction through a vacuum line, such as is the case in the Anstadt device, wrapping the device in a passive mesh which may be fitted around the heart, by suturing or by some form of adhesive. Whichever means is employed, there is a risk of damage to the heart and in particular to the coronary circulation which is made up of a network of blood vessels that traverse the outer surface of the heart.
In International Application No. PCT/AU98/00433 (WO 98/55165) entitled “Cardiac Assist Device”, a device comprising a cup and an internal diaphragm wherein at least a portion of the diaphragm is made from a biointegrating material is described. This device is designed to maximise affixation of the device to the heart by enabling vascularised tissue infiltration into the device. Preferably, the biointegrating material of the diaphragm integrates with the surface of the heart muscle to such an extent that a vacuum or other such means of securement is not required. It is believed that the use of a biointegrating material on the surface of the diaphragm minimises the risk of infection, and rejection of the device by the host's defence system. The device is reliant, however, on a bulky, cup-like structure and requires traditional surgical technique for placement. Such devices may also constrict the heart causing impairment of its filling and proper relaxation. This may also impede blood supply to the heart muscle via the coronary circulation.